Batch Process Control

1. Introduction

2. Sequential and logic control

3. Control during the batch

4. Run-to-run control

5. Batch production management

Introduction

Batch operation is very common in the specialty chemical, pharmaceutical and materials processing industries

Multiproduct batch plants produce a range of similar products using the same equipment

Batch control is particularly difficult because there is no steady-state operating point and processes operate over a wide range of conditions

Batch process control systems involve sequencing of different process steps and control during the batch

Run-to-run control uses experience from previous batch runs to improve the current run

Batch plant scheduling and planning are used to manage the production of different products

Batch Distillation Example

Batch Control Systems

Sequencing and logic – sequencing of control steps according to a predefined recipe

Control during the batch – tracking of predetermined setpoint trajectories

Run-to-run control – end-of-batch measurements are used to improve control of the next batch

Batch production management – planning of production needs and scheduling of product campaigns

Sequential and Logic Control

Control of batch unit operations involves the proper sequencing and execution of specific steps

Sequential logic can be represented in several ways, such as information flow diagrams

Information flow diagrams can be translated into digital logic diagrams to be executed by programmable logic controllers (PLCs)

PLCs can execute sequencing operations are well as standard PID control algorithms

PLCs are the key control hardware used in batch plants

Batch Sequence Control Example

Control During the Batch

Execution of sequenced operations often involves feedback control of specified outputs

Batch control is challenging because there is no steady-state operating point and processes operate over a wide range of conditions

Tracking of predetermined setpoint trajectories is often required

Process nonlinearities and model inaccuracies become more pronounced when processes are operated over large regimes

Some product properties affected during a batch are irreversible once off target

Batch Reactor Control

PI Control of Batch Reactors

t

I

c deteKutu0

zero

)(1

)()(

t

I

c deteKutu0

preload

0 )(1

)()(

Standard PI Controller PI Controller with Preload

Rapid Thermal Processing

Rapid Thermal Processing (RTP) is semiconductor manufacturing process in which silicon wafers are heated to very high temperatures (~1,200°C) on a timescale of seconds.

During cooling temperatures must be reduced slowly so the wafer does not break due to thermal shock.

Rapid heating and cooling rates are attained with high intensity lamps or lasers.

A key challenge in RTP is accurate measurement and control of the wafer temperature during a batch cycle.

High temperature ramp rates prevent the wafer from coming to thermal equilibrium with the process chamber where temperature is more easily measured.

Gain Scheduled PID Control

PID controllers usually provide satisfactory control near the steady-state operating point where the controller was designed

If the process is highly nonlinear and/or operates over a large regime a single set of PID tuning parameter may be inadequate

RTP processes operate over a very wide range of temperatures and exhibit strong nonlinear behavior

Gain scheduling is a simple method to adapt PID controllers to nonlinear processes

Gain scheduling is implemented by adjusting the controller gain according to the current operating point

IMC PI tuning example

I

c

cK

Ks

KsG ,

1)(

Comparison of PID Controllers

Standard PID Controller Gain Scheduled PID Controller

Run-to-Run Control

Product quality measurements are usually collected only at the end of each batch run.

Run-to-run control uses quality measurements from previous runs to adjust conditions of the current run.

Semiconductor manufacturing

» Controlled variable: film thickness

» Manipulated variable: heating rate

Polymer manufacturing

» Controlled variable: molecular weight

» Manipulated variable: chain transfer agent addition rate

Batch Production Management

Many batch plants produce multiple products for multiple customers

Specialty polymers plant: ~25 products for ~100 customers

Planning – procedures to optimally order the raw materials, manufacturer the products, and ship the products to customers

Scheduling – procedures to optimally implement the manufacturing plan by scheduling the use of available equipment

Batch Scheduling and Planning

DETERMINE

What

Product amounts: lot sizes, batch sizes

When

Timing of specific operations, run lengths

Where

Sites, units, equipment items

How

Resource types and amounts

GIVEN

Product requirements

Time horizon, demands, starting and ending inventories

Operational steps

Precedence order, resource utilization

Production facilities

Types, capacities

Resource limitations

Types, amounts, rates

Multiproduct Batch Plant Example

Batch Plant Scheduling Example

Integrated Batch Control Systems